1. Field of the Invention
The present invention relates to power semiconductor devices such as inverters.
2. Description of the Background Art
FIG. 13 shows a conventional power semiconductor device. In this power semiconductor device, an electrode 1a disposed on the bottom surface of a semiconductor device 1, such as a diode, is fixed by soldering on a wiring pattern 3 in the top surface of a ceramic substrate 2. The ceramic substrate 2 is fixed by solder 8 on a base plate 4 composed of for example oxygen free high conductivity copper.
A casing 5 is screwed on or adhered to the base plate 4. The casing 5 is formed by for example PPS (polyphenyl sulfide). Lead frames 6, 7 and a nut 12 are integrated in the casing 5. One ends of the lead frames 6 and 7 function as internal electrodes 6a and 7a, and the other ends function as external electrodes 6b and 7b, respectively. The nut 12 is disposed for connecting external wiring (not shown) to the external electrodes 6b and 7b. The use of a bolt (not shown) that connects by fastening pressure the external wiring to the external electrodes 6b and 7b reduces contact resistance therebetween.
The internal electrode 6a is connected via an aluminum wire 9 to an electrode 1b disposed in the top surface of the semiconductor device 1, and the wiring pattern 3 on the ceramic substrate 2 is connected via other aluminum wire 9 to the internal electrode 7a. For example, the aluminum wires 9 have a diameter of approximately a few hundreds μm.
The internal space surrounded by the casing 5 and base plate 4 is filled with a silicone gel 10 to ensure insulating characteristic. A cover 11 to protect the silicone gel 10 against external air is disposed so as to cover the surface of the casing 5.
In the power semiconductor device shown in FIG. 13, the aluminum wires 9 are used as wiring. In order to reduce the resistance loss when current flows through the aluminum wires 9, it is necessary to increase the cross section area of the aluminum wires 9 (i.e., to increase the wire diameter), alternatively, to increase the number of wires. Joining the aluminum wires 9 to each electrode requires a joint area that has a width of at least not less than twice the diameter of the aluminum wires 9 and a length of at least not less than third times the diameter. If desired to reduce the area of the semiconductor device 1 by increasing the amount of current per unit area of the semiconductor device 1, it is necessary to ensure a wide joint area, which obstructs downsizing of the semiconductor device 1.
In addition, there is the problem that the joint interface between the top surface of the semiconductor device 1 and the electrode 1b is subject to separation due to temperature cycle (heat cycle). When the semiconductor device 1 is made of silicon, its coefficient of linear expansion is approximately 2.3×10−6 [/K]. On the other hand, the coefficient of linear expansion of aluminum is approximately 23×10−6 [/K]. Due to this difference in the coefficient of linear expansion, thermal stress is caused by the exothermic during the use of the power semiconductor device, and the thermal stress induces separation on the joint interface. This thermal stress occurs cyclically when the load on the power semiconductor device is increased and decreased or the power is turned on and off.
For instance, if there is a temperature difference of 50° C., the aluminum wires 9 suffer from separation in a temperature cycle of millions of times. It is therefore necessary to avoid that the temperature of the semiconductor device 1 varies greatly depending on load circumstances in the power semiconductor device.
When plural wires are used for a single joint, current is fed in an island shape from the joint area of each wire to the electrodes, which causes the following problem. That is, the portions of the electrode located in the vicinity of the wire joint areas are low in resistance, whereas to the portions apart from the wire joint areas, resistance component is added to increase resistance loss. A large resistance loss on the electrodes in the surface of the semiconductor device 1 has presented difficulties in obtaining satisfactory device characteristics.
Consequently, if desired to increase the current density of the semiconductor device 1, it is necessary to take the following measures: (i) increasing the size of the semiconductor device to decrease its exothermic density; or (ii) connecting the semiconductor devices in parallel to decrease the exothermic density per device. With these measures, however, the entire size of the power semiconductor device is increased thereby to increase its manufacturing cost.
It also takes from one to a few seconds to joint a single aluminum wire 9. Accordingly, it takes a great deal of time to manufacture a high power module requiring millions of wires, which also increases its manufacturing cost.
FIG. 14 shows other conventional power semiconductor device different from that in FIG. 13. In this power semiconductor device, an electrode 1a disposed on the bottom surface of a semiconductor device 1 is fixed by soldering on one end 7a of a lead frame 7 that is made of copper alloy. The other end 7b of the lead frame 7 functions as an external electrode. An electrode 1b disposed on the top surface of the semiconductor device 1 is connected to one end of a lead frame 6 via an aluminum wire 9. The other end 6b of the lead frame 6 functions as an external electrode.
The semiconductor device 1, aluminum wire 9, and parts of lead frames 6 and 7 are integrally covered by means of transfer molding using a mold resin 14.
The use of transfer moldering facilitates the manufacturing steps of the power semiconductor device of FIG. 14. However, the use of the aluminum wire 9 causes the same problem as in the power semiconductor device of FIG. 13.
FIG. 15 shows still other conventional power semiconductor device. In this power semiconductor device, as in the instance of FIG. 13, an electrode 1a disposed on the bottom surface of a semiconductor device 1 is fixed by soldering on a wiring pattern 3 in the top surface of a ceramic substrate 2. The ceramic substrate 2 is fixed by solder 8 on a base plate 4. A casing 5 is screwed on or adhered to the base plate 4.
The power semiconductor device of FIG. 15 uses no aluminum wire 9. Instead of that, lead frames 6 and 7 made of copper and integrally formed in the casing 5 extend to the vicinity of the semiconductor device 1 and wiring pattern 3, respectively. An internal electrode 6a of the lead frame 6 is connected to an electrode 1b on the surface of the semiconductor device 1, and an internal electrode 7a of the lead frame 7 is connected to the wiring pattern 3. The internal electrodes 6a and 7a are connected via a conductive adhesive to the electrode 1b and wiring pattern 3, respectively.
The use of the lead frames 6 and 7 in place of aluminum wire avoids increasing the entire size of the power semiconductor device and its manufacturing cost that have been problems involved in using aluminum wire.
However, another problem remains in the power semiconductor device of FIG. 15. The problem is differences in the coefficient of linear expansion between the semiconductor device 1 and lead frame 6. When the semiconductor device 1 is made of silicon, its coefficient of linear expansion is approximately 2.3×10−6 [/K]. On the other hand, the coefficient of linear expansion of copper that is the material of the lead frame 6 is approximately 16.7×10−6 [/K]. Due to this difference, thermal stress is caused by the exothermic during the use of the power semiconductor device. Due to this thermal stress, the connection part of the lead frame 6 is subject to fatigue failure. In other words, there occurs separation of the conductive adhesive. Even if soldering is employed in place of the conductive adhesive, the problem of fatigue failure still remains.